Letting chicks out from coop at what age???

I am totally new to having chickens! I just got my first 9 about 3 weeks ago and have a question. I have 5 chickens (pullets we were told!)that are a little over 3 weeks old and 4 that are a little over two weeks. I am wondering how long we have to wait before we let them actually run around in the run/yard. We have put them in their coop so they get accustomed to it but have yet to let them roam freely in the run. I considered letting them out while I sit out there for a bit to see how they make out but I am not sure they should be out alone as of yet. I have looked up info but get many different answers so I am not sure what to do.

I also have been having the issue of them getting to hot while they are "cooped" up in the coop (no pun intended). We have had a week of 90 and above heat this week and I have had to remove them from the coop and put them in the only other option I have and that is a wagon with chicken wire over the top so they stop gasping with their beaks open! Is there another option, or am I being overly worried about them. I am worried they will be too hot in summer but I guess at that point they will be outside so it may not be the same circumstance. Any advice or help is appreciated!!

My concern with letting them out alone would be predators. If your run is not covered, you will be dealing with cats, crows, hawks, and jays entering the run looking for a quick meal.

The coop has to be getting really hot for them to be overheated as young as they are. You may want to look into adding more ventilation to the coop. It is hard to make specific recommendations with seeing your set up.